The invention relates to a method for applying an absorber layer onto a central tube. The invention furthermore relates to a central tube for a linear concentrating solar thermal power.
Linear concentrating solar thermal power plants are generally known. They can be configured as a parabolic trough power plant. The latter have parabolic trough collectors, which have a parabolic cross section which is upwardly open such that sunlight can be incident. The rays of the sun are reflected by the inner surface of the trough and impinge on a central tube, in which a heat-storing fluid circulates (for example an oil or molten salt). The heat transfer medium in the central tube transfers the heat for example to a thermal power generation system. Fresnel collectors can alternatively be used, in which the sunlight is collected using a Fresnel lens rather than with a parabolic mirror. The concentrating solar thermal power plants will also be referred to below as solar power plants for short. It should furthermore be noted that the term power plant should be understood in the widest sense. It is not only possible to equip heat engines such as turbines with the abovementioned collectors, but the heat stored in the heat-storing medium can also be used directly.
In order to achieve as high an efficiency of the solar power plant as possible, as much of the solar energy as possible must be injected into the heat transfer medium. To this end, the central tube is provided with a spectrally selective absorber layer which has as high a solar absorptivity α as possible and as low a thermal emissivity ε as possible. In order to efficiently convert solar light energy into thermal energy, the absorber layers should generally have a high solar absorptivity α and a small thermal emissivity at the respective operating temperature. Spectrally selective absorber layers are therefore characterized by low reflectivity (ρ≈0) for wavelengths λ under 2 μm and high reflectivity (ρ≈1) for wavelengths λ over 2 μm. Therefore there is a need for spectrally selective absorber layers for linear concentrating solar thermal power plants, which are easy and cost-effective to produce and are chemically and thermally stable at temperatures of around or over 500° C. in air. An ideal absorber layer would have a solar absorptivity α≧0.98 and a thermal emissivity ε≦0.05 at at least 500° C. For operation at what is referred to as medium temperature, a solar absorptivity α>0.85 and a thermal emissivity ε<0.15 at 400° C. are aimed for in the technically customary realization of absorber layers. The stability of the absorber layer in air under such operating conditions is another objective because it renders vacuum encapsulation of the central tubes in parabolic trough collectors, which is still prevalent today, requires great outlay in terms of construction and is expensive, obsolete.
It may therefore also be sensible to tolerate lower solar absorptivities α and/or higher thermal emissivities ε than those mentioned above if, in return, the air stability under the desired operating conditions can be attained in an economically favorable manner. Such coatings, which are cheap to produce, are known from commercial parabolic trough systems for operating temperatures of up to 300° C. (SOLERA sunpower), which are mainly designed to generate process heat and not to generate power. However, they are not suitable for operating temperatures that are much higher, for example 400° C.
According to U.S. Pat. No. 4,005,698 it is known to generally provide surfaces as broadband converters for light energy, which have a dendrite-type surface structure. It is important here that the dendrites in terms of their height and their spacing are in the region of the wavelength or of a low-number multiple of this wavelength in order to ensure optimum absorption of the light at the relevant wavelength. The dendritic layers can be applied according to U.S. Pat. No. 4,005,698 by vapor deposition.